The aging mouse exhibits a severely depressed immune response. Most of the data in support of this statement have been acquired in aging virgin mice. However, recently we have shown that with respect to two major parameters of the immune response, macrophage activation by adjuvants and antibody formation, that although aging female virgin mice are poor responders, aging female breeder mice have approached young adult levels of activity. Consequently, this study will explore the means by which aging breeder mice respond to immune stimuli and aging virgin mice do not with experiments testing (a) the time span over which the defect in immunity is acquired by the virgin mice, (b) the levels of interleukin I and II in aging breeder and virgin mice, (c) whether levels of prolactin, estrogens and testosterone in the two groups of aging mice can be correlated with the immune response differences, (d) whether the macrophage and antibody response of aging virgin mice can be restored by cytokine supplementation.